1. Field of the Invention
This invention relates to a photovoltaic device for converting optical energy into electric energy and more particularly to a photovoltaic device which is improved in resistance to thermal degradation of the conversion efficiency without deteriorating the initial conversion efficiency.
2. Description of the Prior Art
A photovoltaic device can be composed mainly of amorphous silicon deposited from a gas containing a silicon compound such as SiH.sub.4, Si.sub.2 H.sub.6 or SiF.sub.4. Although such a photovoltaic device composed mainly of amorphous silicon can be manufactured with its larger area at lower cost and thus is preferable as a solar battery, it is largely degraded in the conversion efficiency with the passage of time. It is known that there exist two types of degradations in the conversion efficiency with the passage of time, i.e., one is optical degradation caused by intense light irradiation and the other is thermal degradation caused at higher temperatures (refer to the Conference Record of the Eighteenth IEEE Photovoltaic Specialists Conference, 1985, pp. 1712-1713, Las Vegas, U.S.A.)
Incidentally, each of the U.S. Pat. Nos. 4,476,346 and 4,388,482 describes a photovoltaic device composed mainly of amorphous semiconductor film which includes a doped layer of hydrogenated amorphous silicon nitride (a--SiN:H) neighboring on a metal electrode. However, this doped layer is a single layer and does not include such sub-layers as will be described in the following.